In his article entitled "Solitary Waves", which was published in American Scientist, Volume 80, pages 350 to 361, 1992, Russell Herman provides a review of the phenomena known as "solitary waves", which were termed "solitons" by N. J. Zabusky and M. D. Kruskal following their work with computer simulations of the interactions of solitary waves in the 1960s. In the concluding section of his article, Dr. Herman notes that in the field of optical communications, electromagnetic solitons may be used in fibre optic transmissions, but admits that no practical fibre-optic system has been produced and that, in other areas, electromagnetic solitons remain a subject "for theoretical investigations and potentially practical applications".
The present invention represents a practical application of solitons in what are known as defocusing non-linear media, such as polymers; sol-gels; glasses or crystals doped with non-linear molecules which provide thermal or electronic non-linearities; semiconductors; and semiconductor-doped glasses. (This list is not exhaustive.)
In an optically transparent defocusing non-linear medium, dark solitons exist as regions of low light intensity contained within a higher intensity background. They are formed when an intense quasi-plane wave (such as a wavefront produced by a laser) is incident upon the medium and a suitable perturbation is applied to the propagating wavefront. Suitable perturbations include (a) one-dimensional and two-dimensional phase perturbations, (b) one-dimensional and two-dimensional amplitude perturbations, and (c) combinations of both amplitude and phase perturbations. In the case of phase perturbations, the local phase of the quasi-plane wave changes across a boundary, or boundaries, or at some point, within the wavefront. To effect an amplitude perturbation of the wavefront, the quasi-plane wave may be passed through a partially or totally occluding mask.
Spatial solitons are formed in the defocusing medium by the non-linear interaction between the propagating wavefront of the beam and the medium. The solitons appear as dark regions (such as stripes, grids, dots or rings) in the light beam after some distance of travel, hence the adoption of the term "dark solitons". The light intensity in the darkest region of a soliton can approach zero, in which case the soliton is termed a "black soliton". Other dark solitons are called "grey solitons".
The dark solitons propagate without diffracting. For this to be possible, the non-linear medium must have a negative value of its third order non-linear susceptibility (that is, the refractive index of the medium must decrease with increasing light intensity). Black solitons always propagate parallel to the wave-vector (k-vector) of the soliton-forming beam. Grey solitons propagate at finite angles to the wave-vector.
The decrease in the refractive index of the non-linear medium when a soliton-forming beam is projected into it can be fast or slow. In the former case, there is an instantaneous change in the refractive index in response to changes in the intensity of the soliton-forming beam. If, however, the decrease in the refractive index of the medium is slow, the refractive index responds to the total energy delivered by the soliton-forming beam, integrated over an appropriate interval. Furthermore, the change in refractive index can be transitory (occurring only while the soliton-forming beam is present), or it may be permanent (in which case the change in refractive index in the medium remains after the soliton-forming beam is no longer present).
A refracting structure is created in the non-linear medium by the dark solitons. Since the refractive index of the defocusing non-linear medium decreases with increasing light intensity, the refractive index in the darkest region of the solitons is higher than in the brighter regions of the beam surrounding it. The refracting structure thus formed in the vicinity of the soliton acts to guide the soliton-forming beam in a manner similar to that of an optical fibre or an ordinary linear slab waveguide. In the soliton case, however, the soliton-forming beam itself induces the waveguiding structure. Thus solitons can be thought of as self-guided waves with the specific condition that the soliton is a mode of the waveguide that it creates. However, in comparison with optical fibres (where the radiation is confined within the waveguide), for a dark soliton, the radiation in the soliton-forming beam is located outside the waveguide. Hence, whereas the guided beam in an ordinary optical fibre is a bound mode of the waveguide, dark solitons are equivalent to radiation modes of the waveguide which the field induces.
The present inventors, in their paper entitled "Waveguides and Y Junctions formed in Bulk Media by using Dark Spatial Solitons", which was published in Optics Letters, volume 17, pages 496 to 498, 1992, have described how dark spatial solitons can be used to form adiabatically tapered waveguides, and waveguide Y junctions. The disclosures in that paper are incorporated into this specification by this reference to that paper. At the time of writing that paper, the present inventors commented upon the potential to use optical waveguides induced by dark solitons as "light-controllable structures", but they presented no indication of how such practical structures could be created.